As background technique in the present technical field, Japanese Patent Application Laid-Open No. Hei 10-310017 (Patent Literature 1) is known. In this gazette, a vehicular seat in which an expanding direction of a side air bag from a seat back is specified is described (see the abstract). In this vehicular seat, the side air bag is fixed, via an air bag bracket, to a side bracket of a seat back frame. Further, the side air bag is surrounded with webbing (backing cloth) provided to form a gap with respect to the side bracket, in an approximately horizontal direction. The both ends of the backing cloth are individually sewn (joined) to the front end of a gore part and the side end of a main part of a trim cover. Further, sewn ends of the backing cloth are sewn (joined) to each other as a sewn part capable of being torn by expansion of the side air bag.